A vehicle brake master cylinder provides control and routing of brake fluid flow to the individual wheel brakes. The master cylinder is typically provided with primary and secondary pistons, each providing brake fluid at an operator determined pressure from a separate portion of a brake fluid reservoir to a separate pair of brake mechanisms.
A particular function performed by the master cylinder is the control of fluid as required between each portion of the brake fluid reservoir and its assigned wheel brakes. This is accomplished with the assistance of bypass and compensation openings through the walls of the master cylinder housing between the brake fluid reservoir and the primary and secondary pressure chambers of the master cylinder. Bypass openings are located immediately in front of the primary seals on the primary and secondary pistons so that they are open to the appropriate wheel brakes with the pistons in their deactivated positions; and each thus permits replenishment of fluid in the brake lines in front of its associated piston to fill the volume between the piston and brake apply mechanisms in preparation for the next braking activation. Each bypass opening is passed by its associated primary seal almost immediately with activation of the piston to trap fluid in the brake lines and thus permit increase of braking pressure with further advance of the piston. A compensation opening is located behind the primary seal of each piston, even in the deactivated position of the piston, and maintains fluid volume behind the primary seal to prevent air from being pulled across the seal into the pressure chamber.
Such master cylinders may be used on vehicles provided with traction control or other systems capable of activating wheel brakes independently of the master cylinder and having a pump with an inlet open to the brake lines downstream of the master cylinder to provide brake fluid to the wheel brakes at the required pressure. In some modes of operation, such a pump can temporarily remove fluid from the brake lines at a rate which is greater than the flow capacity of the bypass opening, and/or the compensation opening across the seal, to replenish the fluid. In such vehicles, improved ability to flow fluid from the reservoir, through the master cylinder, to the traction control pump would help provide improved traction control response time. But it is not desirable to enlarge the bypass openings to provide the desired improved fluid flow, because a larger opening has a greater tendency to wear out or damage the seal that repeatedly slides over it. In addition, the seal construction tends to require a compromise between wear life and the ability to pass fluid downstream past the seal when required as described above; and a change in seal construction to permit greater fluid flow from the compensation opening past the seal into the brake lines may also cause a reduction in seal wear life.